Numerical predictions of flashback limits of H2-enriched methane/air premixed laminar flames

Hydrogen is considered as a promising resource for decarbonizing not just the industrial sector but also domestic heating systems. By partially substituting natural gas with hydrogen, domestic combustion-based conversion systems have the potential to enhance efficiency, decrease carbon emissions, and achieve cleaner combustion, specifically reducing levels of particulate matter. Nevertheless, hydrogen possesses properties that differ significantly from natural gas. In particular, due to its higher laminar flame speed, hydrogen has a much higher propensity to flashback than natural gas, raising notable safety concerns. This study aims to examine the impact of H2 addition (up to 100%) to natural gas on the combustion process in domestic condensing boilers. To achieve this objective, 3D numerical simulations are conducted, modeling the multi-hole geometry that emulate perforated burners commonly found in these appliances. The simulations incorporate detailed kinetics and conjugate heat transfer with the burner plate and consider various hole-to- hole distances for a more comprehensive analysis. Flashback limits are found for a wide range of operating conditions of interest for domestic applications, with equivalence ratios from 0.5 to 1 and hydrogen fractions from 0 (pure methane) to 1 (pure hydrogen). The results confirm the observations of previous works on planar, multi-slit configurations. More specifically, the results shows that the conventional flashback correlation based on the concept of critical velocity gradient becomes inaccurate for H2 fractions larger than 0.50 as it does not take into account stretch induced preferential diffusion effects, which are especially large in the multi-hole configuration here investigated.